A bookbinding system is known in which a plurality of sheets on which images are formed by an image forming apparatus is bundled into a sheet bundle, and an adhesive is applied to a spine of the sheet bundle to produce a complete book.
Thus, JP-2008-55677-A discloses a bookbinding system that forms an adhesive toner image on a binding edge portion of each sheet using the image forming apparatus. Specifically, in an image forming unit of the image forming apparatus, a regular toner image is transferred to the sheet based on image data as well as the adhesive toner image is transferred to a portion corresponding to the spine edge of the sheet. The sheets on which these toner images are formed are conveyed to a fixing device, and are sequentially sent to a bookbinding processor after the toner images are fixed onto each sheet. The plurality of sheets conveyed to the bookbinding processor forms a sheet bundle stacked on a stack section. Next, a spine edge portion of the sheet bundle is coated with an adhesive heated by a heater. The adhesive toner image is heated and softened by the heated adhesive to thus serve as an adhesive to attach adjacent sheets to each other. As a result, the adjacent sheets are attached to each other by the adhesive toner image and the adhesive.
However, in the bookbinding system as disclosed in JP-2008-55677-A, because the adhesive toner image once heated and softened by the fixing device and fixed onto the sheet and the adhesive coated on the spine portion of the sheet bundle are used to attach the adjacent sheet to each other, some means for applying the adhesive on the spine portion of the sheet bundle is required, which increases the cost of the apparatus. However, if binding of the sheet bundle is performed using the adhesive toner image alone after fixation, the adhesive force is weak and sheets are separated easily from the booklet.